The present invention relates to an imaging optical system, and particularly to a new and useful way of producing control information for the imaging optical system. The present invention is particularly useful in providing control information for an imaging optical system such as a lithographic imaging optical system.
In U.S. application Ser. No. 11/544,833, filed Oct. 5, 2006, assigned to the assignee of the present invention, and incorporated by reference herein, there is disclosed a system providing control information for an imaging optical system such as a lithographic imaging optical system. In that system, imaging optics define a primary optical path along which a primary image is projected (e.g. onto a wafer), and a measurement optical path is established and includes at least part of the primary optical path. The imaging optical system is configured to obtain information from the measurement optical path for use in providing control information for the imaging optical system. The system includes, e.g. optics, detectors, electronics, and mechanics etc., which detect the information from the measurement optical path, and produce control data that are useful in the imaging optical system. The metrology features that are provided by the system of that application are sometimes referred to by applicants as “through the lens” metrology, because the measurement optical path, in those cases, is at least partially through the primary optical path.
The principles of U.S. patent application Ser. No. 11/544,833 are primarily useful in controlling positioning of the wafer in directions that are toward and away from the imaging optics.